The force that opposes relative motion between two touching surfaces is friction.
Friction acts at the interface between surfaces and resists their relative sliding, or the sliding that would occur without it. It is caused by microscopic surface irregularities, adhesion between materials, deformation, and other interactions at the contact region. Despite its reputation as a nuisance, friction is essential: it lets people walk, lets tires grip roads, and allows brakes to slow vehicles.
Static friction acts when surfaces are not sliding relative to each other. It adjusts from zero up to a maximum value, commonly modeled as Fₛ ≤ μₛN. Once sliding begins, kinetic friction usually applies and is modeled approximately as Fₖ = μₖN. The normal force is perpendicular to the surfaces, while friction acts parallel to them.
Drag is also a resistive force, but it arises from motion through a fluid such as air or water. Tension acts through ropes or cables, and normal force supports surfaces. A subtle point is that static friction can point in the direction an object is moving, as with a car tire pushing against the road; it opposes slipping, not necessarily the object’s overall motion.